polyvinyl acetate adhesives forestry 485 lecture 3-4

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