IMUTHANE Processing. Processing of Castable Polyurethane Elastomers – Basic Considerations Mobile...
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Transcript of IMUTHANE Processing. Processing of Castable Polyurethane Elastomers – Basic Considerations Mobile...
IMUTHANE Processing
Processing of Castable Polyurethane Elastomers – Basic Considerations
Mobile liquids (2 or more parts) Accurate ratio control needed (ratios range from 1:1 to
3:100) Good mixing Accurate temperature control at all stages Moisture exclusion Good degassing Proper molds and filling techniques Finishing techniques
Prepolym er Curative
Polyurethane Prepolym er Processing
Dispensing
Dem old
Curing
Finishing
Post-Cure
(M elting),W arm ing &Degassing
M elting orW arm ing
(Degassing)
M etering
M olding
M ixing
Step Do’s Don’tsDegassing Degas Prepolymers
Maintain Proper Vacuum and Temperature
Try to get by Without DegassingTrust Gauges without Checking
Do’s and Don’t’s of Urethane Processing III
Processing – Degassing Degassing removes dissolved and absorbed gases, from
manufacture and those generated through natural aging Viscosity and surface tension of the prepolymer entrap air
and other gases Degassing of the prepolymer is carried out at the
processing temperature: 80 – 100°C Degassing of the prepolymer is carried out in a vacuum of
28.5” Hg
Processing – Degassing Degassing is complete when vigorous foaming stops – the
time required is dependent on: the amount of prepolymer the temperature of the prepolymer the size and shape of the vessel the degree of vacuum the amount of entrapped gases
Use a container that is twice the volume of prepolymer to be degassed to allow for foam rise
It may be necessary to re-heat the prepolymer after degassing because of heat loss
Step Do’s Don’tsPrepolymer/Curative Gas Blankets
Use Dry Gas (N2, Dried Air) Use Ambient AirUse Plant AirLeave Containers open
Curative Transfer
Use GlovesUse RespiratorObserve Safety RegulationsWash up
Use Bare HandsSpill Materials
Metering Check CalculationsCheck for Proper Processing ConditionsUse Proper Size BalanceRecord Weights and Conditions
Weigh Without CalculationUse Paper Cups Guess at Conditions
Do’s and Don’t’s of Urethane Processing IV
Processing – Prepolymer Preparation
Refer to Technical Datasheet for specific processing requirements
Heat only the required amount of prepolymer to the processing temperature
Add pigment, anti-foam, other additives Degas at the processing temperature, usually 80 – 100°C Reheat to processing temperature if required
Processing – Curative Preparation
Refer to Technical Datasheet for specific processing requirements
Weigh the appropriate amount to cure the prepolymer, allow for residual
Heat to the required processing temperature, if required (e.g. MBCA)
Curative is always added in liquid form to hot prepolymer
Mix Ratio / Stoichiometry
What is Stoichiometry? It is the ratio of reactive groups in the curative to
reactive groups in the prepolymer It is represented by % Theory
The best mix of properties is found between 95 and 100% Theory
Outside of this range there can be significant impact on the physical properties
Mix Ratio Calculation %NCO is the weight per cent of reactive isocyanate groups
in the prepolymer. It can be found on the drum label or CoA for each batch.
EW is the equivalent weight of the curative. It can be found in the Technical Datasheet. For GR133.5, EW = 133.5 For GR107, EW = 107
Calculation:
4202
%Theory Desired x EW x %NCO Curative pph
Mix Ratio Calculation – Example 1 IMUTHANE 22-90A cured with GR107
22-90A: %NCO = 4.15%. GR107: EW = 107 Desired %Theory: 95%
Calculation:
4202
%Theory Desired x EW x %NCO Curative pph
4202
95 x 107 x 4.15
10
Mix Ratio – Effect on Tensile Strength & Elongation
Mix Ratio – Effect on Tear Strength
Mix Ratio – Effect on Compression Set
Do’s and Don’t’s of Urethane Processing V
Step Do’s Don’tsMixing Make Sure Temperatures Are Correct
Mix Quickly and ThoroughlyScrape Sides and BottomUse StopwatchProvide VentilationWear Safety Glasses and GlovesClean Tools After Mixing
Whip in AirExpose to MoistureUse Wooden SticksBreathe FumesGuess at Mixing Times
Processing – Mixing Check to ensure correct temperatures before mixing Mixing can be achieved:
Using a plastic container & spatula Using a drill with attachment By machine
Thorough mixing by hand is achieved by scraping the sides and the bottom of the container and using a figure-8 stirring pattern
Mix quickly and thoroughly and avoid entraining bubbles Pour mixing can cause poor physical properties Degas after mixing if time permits (hand mixing) Remember – safety – ventilation, gloves, eye wear, protective
clothing
Mixing Issues – Curatives
Curatives like IMUCURE GR107 are Liquid and very easily mixed into prepolymers.
Do’s and Don’t’s of Urethane Processing VI
Step Do’s Don’tsPouring & Casting
Pour Evenly and avoid SplashingAllow Trapped Air to Sweep outControl Temperatures (mold and Material)
Dump Material in to Save TimeFlood the Pour SpaceMove Molds too Quickly
Cure and Postcure
Control TemperaturesUse Correct Time
Use Cold Molds or InsertsUse Excessively High Temperatures
Processing – Pouring When the prepolymer and curative are mixed, the mixture is
poured into the hot mold for curing Pouring should be carried out in a way that minimises
turbulence and stops bubble entrapment in the mold Pour evenly and avoid splashing Other molding techniques include:
Injection Rotational Centrifugal Vacuum
Processing – Temperatures
Material temperature – what does it affect? Demold hardness and toughness Cure consistency Efficiency of production Efficiency of mixing Ratio – temperature effects viscosity which can change back
pressure in machines, and cause mixing difficulties if hand mixing
Processing – Temperatures
Mold temperature – what does it affect? Demold hardness and toughness Cure consistency – incorrect mold temperature causes
Shrinkage Voids, cracks Sink marks, “christmas tree effect”
Efficiency of production Bonding for to metal.
Processing – Molds Mold temperature – refer to Technical Datasheet for material Molds should be coated with a release agent to ensure easy
removal of cast parts Take care in handling molds – any damage to surfaces will be
reproduced in the polyurethane part Wide material choice for making molds – steel, aluminium,
polyurethane, silicone rubber, … Mold material must be dimensionally stable, heat resistant and
inert to the chemical reaction
Processing – Cure & Post-Cure
Material in the mold must be allowed to cure before demolding – the time required depends on the material and varies from 10 minutes to several hours.
Material is then post-cured, generally for 16 hours at the specified temperature. Time and temperature will depend on the material.
Post-cure is necessary to develop physical properties. Full property development occurs over the next 7 days at
ambient conditions.