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 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 nature
High temp. softens adhesive
Creep
Moisture compromises performance
Under shear PVAc bonds as strong as maple
Perpendicular grain joints are allowed to “flex”
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
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 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
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
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 panel
B. 55 gallon drum of water on same panel
C. Same drum hung from lever with 4:1 advantage
A. 1.39 lb/in2
B. 3.17 lb/in2 Load = 55x8.3 = 456.5 lbs
C. 12.68 lb/in2 Load = 456.5x4 = 1826 lbs
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 shopscale 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