Polyurethanes - Processing, Health and Safety

Topics Covered

Thermoset PUs

One Step Process

Two Step Process

Production of PU Products

Foams

Moulded Foam

Continuous Slabstock

Rigid Foam Production

Thermoplastic Polyurethanes

Health and Safety and Polyurethanes

Inhalation

Eye Irritation

Skin Effects

Thermoset PUs

There are two main processes for thermoset PUs.

Many PUs, particularly the foamed products, utilise a one step (one shot) process; a two stage process based on the pre-polymer system, is used in the manufacture of elastomers, paints, adhesives and sealants.

One Step Process

In this system the polyurethane is manufactured from liquid isocyanate and polyol using a purpose designed machine. The blend instantly reacts as the mixture is injected into the mould or, for coatings, sprayed onto the product. The high reactivity of the system causes a high exotherm and consequently rapid curing.

A dispensing machine monitors and controls the temperature of the constituents as required, accurately meters them at selected ratios into a mixing chamber where they are intimately blended and a pre-selected weight of liquid mix is dispensed to the mould or spray head.

The output rate of the machine can be adjusted with respect to the weight of mixed liquid dispensed per second. The dispensing machine must be accurately calibrated and the processing conditions and temperatures must be kept within defined limits.

The process makes particular demands of the operators, who must be trained and expert both in the setting up and use of machinery and the correct storage and handling of the chemicals.

For example once a container of polyol or isocyanate is opened care must be taken to prevent moisture pick up. The recommended practice is to purge the container with nitrogen each time it is opened and resealed.

Two Step Process (Pre Polymer System)

The PU is prepared from a pre polymer, where the polyol blend is reacted with an excess of an isocyanate so that full polymerisation cannot occur, the reaction being completed when the pre polymer reacts with an additional polyol and chain extender see figure 1.

The mixing can be done by hand, in low pressure mixer/dispensers, and in reaction injection moulding (RIM) machines. In the latter operation the degassing operation is not required.

Production of PU products

Foams

PU foams fall into two distinct categories flexible and rigid.

There are two techniques for the production of flexible foam depending on the product.

Moulded Foam

Moulded Foam products such as car seats, furniture and packaging are produced in a one step process by the reaction of the raw materials in a mould using high or low pressure dispensing equipment. Two types of process are used: hot cure foam and cold cure foam.

In the hot cure process external heat is applied to the mould after filling so that there is sufficient cure of the foam moulding to allow early release from the mould. For the cold cure process lower operating temperatures are used.

Although the cycle times are slower there is greater freedom in the choice of materials for mould construction.

Continuous Slabstock

Continuous slabstock is again a one step process using high capacity mixing and dispensing equipment. The handling of the large quantities of foam requires appropriate automated cutting and conveying systems.

Rigid Foam Production

The continuous production of rigid foam uses equipment very similar to that for producing flexible slabstock foam.

Large quantities of block rigid foam can be manufactured by the proportioning of the ingredients to a mixing head and then dispensing onto a moving conveyor belt. The rigid foam is then cut to the desired shape and size.

This method is also used for the continuous production of laminated or sandwich panels where the reaction mixture is dispensed onto the lower surface of the panel facing and rises to the upper surface. The panel thickness is controlled by setting the distance between the lower and upper conveyor.

For constructional panels of complicated design and varying dimensions the discontinuous production route is used. The foam is injected into a cavity mould in which the panel facings have been placed. The reaction mixture must be precisely dispensed so that the cavity is fully, but not over filled, and, in order to withstand the foam pressure, mould clamps are used.

A similar processing principle is applied when PU foam is used as insulation for refrigerators and freezers. The reactive mixture is injected into the cavity of a preassembled outer casting and inner liner of a cabinet supported in a jig.

Much insulation work is carried out by the use of two component spray equipment. Cosmetically, sprayed rigid PU foam is not generally very good, but the process is very useful when insulating difficult and intricate surfaces. Cavity wall insulation is carried out by the reactive mixture being pumped in to the cavity to be filled. The foam adheres to the surfaces as it reacts and cures.

Reaction injection moulding (RIM) uses high pressure dispensing machines and is really a specialised form of casting. It involves the high pressure mixing of the two liquid components which are then injected into a mould see figure 2.

Figure 2. The RIM process.

The RIM process allows moulded articles to be produced at rates comparable to conventional injection moulding. The process produces a range of self skinned mouldings from both flexible and rigid PUs.

Depending on the reactivity of the PU system and part dimensions, the part can be removed from the mould within 1 to 10 minutes. The moulding pressures are low, typically less than 100 psi, which means lower tooling costs.

RIM machinery can allow fillers to be mixed into the reacting system, yielding products with improved physical properties called Reinforced Reaction Injection Moulding (RRIM).

Cast PU Elastomers are processed in liquid form and, for best results, should be degassed to remove any moisture or air. This is best done by heating to the processing temperature and applying a vacuum.

Mixing may be either batch or continuous followed immediately by pouring or dispensing into a cold or heated moulds. The moulds are left at the appropriate temperature until the part cures to a solid end product which is discharged and trimmed as necessary.

The flow of the reactive chemical exerts very little pressure on the mould so they can be made from inexpensive materials e.g. glass reinforced plastic (GRP). Usually after 3 to 8 minutes the moulding can be removed. However a post moulding cure is often required to develop optimum levels of physical properties. A typical cycle is 40 to 50 minutes at 110°C.

If a cast PU is not sufficiently cured, lower impact, flexural strength and heat distortion temperature may result.

The casting of PUs is a batch process. Small and large numbers can be readily accommodated and the castings from one gram to one hundred kilos may be made by the same process. The solid end products can range from about a 10 Shore A to 90 Shore D hardness.

PU millable gums are processed like conventional rubbers using a Banbury mixer and mill and are cured with peroxide or sulphur.

Both polyether and polyester derived PUs are available, their products are formed by the usual elastomer techniques of extrusion, compression and transfer moulding.

Thermoplastic Polyurethanes - TPUs

TPUs are supplied in granules or pellets for processing by all of the usual thermoplastic processing techniques such as injection moulding, extrusion (for tubing, film and profiles) and vacuum forming.

TPUs are usually supplied in a sealed moisture proof bag since they are, to some degree, hygroscopic and will deteriorate if left exposed to the atmosphere. Ideally, dried TPUs should have a maximum level of 0.02 wt% water, and must be kept below 0.05 wt% water, at which level the reaction with the water starts. TPUs that have been processed whilst wet are total scrap and cannot be recovered.

Of the two major types of TPUs available, polyesters and polyethers. A polyether based TPU is more sensitive to overheating during processing. Typical injection and extrusion temperatures are between 180 to 230°C.

Health and Safety and Polyurethanes

In addition to the usual care associated with the use of any production machinery particular precautions must be taken in storage, handling and processing of PU chemicals.

Isocyanates present a range of potential hazards

Inhalation

Isocyanates will irritate the entire respiratory tract and excessive concentrations could cause difficulty in breathing. It is therefore important to avoid breathing the vapour of any isocyanate and control limits and threshold limit values (TLVs) should be adhered to at all times. The level of hazard varies with different isocyanates. according to their vapour pressure and the temperature at which they are handled, the hazard increasing as the temperature rises. People suffering from asthma and bronchitis, or from an allergic reaction should not work with, or near, isocyanates.

Eye irritation

Isocyanate vapour also causes discomfort to the eyes. Splashes of liquid isocyanate to the eyes will cause mild to severe irritation and should be treated immediately using the contents of a sterile eye wash bottle.

Skin effects

Absorption by the skin is low and slow, but will cause irritation and may cause tanning. Soap and water are usually sufficient to remove small amounts and since the water will react with the isocyanate immediately, the irritant will be neutralised.

With pre polymers the isocyanate vapour pressure is reduced by the partial reaction with the polyol, and therefore they are less hazardous.

It follows that the storage processing and handling of isocyanates and pre polymers must be conducted under proper conditions.

Processing areas must be fitted with ventilation systems and personnel should be trained in handling drums of isocyanate, particularly when drums need to be heated in order to melt the contents. For bulk storage, isocyanates should not be handled in open vessels. Although isocyanates are not particularly flammable, it is recommended that bulk storage should be in a well ventilated area and separated from the work place.

The hazards to health from skin contact may be avoided by the use of proper safety equipment, masks, protective clothing, eye protection and impermeable gloves.

Primary author: Brian Lees

Source: Materials Information Service edited by Stephen Harmer

For more information on the Materials Information Service please visit The Institute of Materials.