FOAMABLE POLYVINYL ALCOHOL COMPOSITION

Abstract: Foamed polyvinyl alcohol can be produced by extruding in the presence of water which acts as a blowing agent. The material is rendered waterproof by incorporating UV-curable resin.

Description: FOAMABLE POLYVINYL ALCOHOL COMPOSITION This invention relates to polyvinyl alcohol compositions which are biodegradable and, hence, environmentally friendly alternatives to polystyrene, polyethylene and other synthetic resins. In one aspect of the present invention, a foamed or expanded product is provided, capable of use in the form of individual pieces as loose fill packaging material, insulation and cushioning material, and in the form of sheets and blocks as insulation, particularly in the construction industry. While foamed polyvinyl alcohol has been proposed in the past, it has never been practical to product in a large scale, particularly by extrusion techniques of the type used for polyalkylenes. In particular, there is a need for a foamed product which does not require formaldehyde in its manufacture We have now devised a process for the production of a foamed polyvinyl alcohol composition comprising the following steps: 1 blending one or more finely divided polyvinyl alcohol formulations, in particular compositions having a mol % hydrolysis figure of 74 to 94, e.g. 80 to 90 for a cold water soluble product or 98 to 100, e.g. about 99 for a hot water soluble product with a minor proportion of glycerol and water, optionally with colouring and opacifying agent or the like, the blend being allowed to reach a temperature of at least 100; 2. passing the blend through a screw extruder and extruding the material at a die temperature of 125 to 240°C, the blend having been saturated with water, either by simultaneously injecting water at an intermediate point in the extruder barrel, or by first soaking the blend in water and then drying the surface before introduction into the extruder. By this method, the water becomes closely absorbed in the blend while it is being treated in the screw barrel under pressure and instantaneously boils on passing the extruder die, thus foaming and expanding the extrudate The polyvinyl alcohol used in step 1 is chosen desirably to be hot water soluble and compostable and thus has a mol % hydrolysis value of at least 98, more particularly a hydroxy content or about 99. It may be desirable to blend two or more polyvinyl alcohol compositions to obtain the correct properties, for example a product within a % mol hydrolysis value of 98% and a viscosity of 56 mPa.s and a product with a mol % hydrolysis value of 99% and a viscosity of 28 mPa.s. The final blend passed to the extruder typically contains from 60% to 70% polyvinyl alcohol, the remaining part (other than colouring or opacifying additives) comprising glycerol and water in the range 2:1 to 3:l preferably about 2.5: 1. Most preferably the blend contains about 65% polyvinyl alcohols, about 25% glycerol, about 10% water and about 0.25% opacifier (such as a finely divided silica or T,O 2 ) with optional colouring agent. The blend should preferably be allowed to reach a temperature of at least 105°C in order to obtain an intimate mixing of the various ingredients which is suitably plasticised and extrudable. It may be desirable to cool the blend and store it before passing it to the extruder. Typically, a temperature of 25°C to 35°C, e.g. about 30°C is suitable. Extrusion can take place in a general purpose screw extruder having controlled zonal heating to give a temperature profile along the barrel. A typical temperature profile is for the barrel to have four zones in sequence running at 70 to 125, e.g. about 100°C, about 120°C, about 130°C and 125 to 240, e.g. about 130°C, leading to an adapter also running at about 130°C leading to the die which should be operated at a slightly higher temperature, typically 135°C to 140°C. A typical apparatus has a 2: 1 compression and should be operated at maximum screw speed. A typical example of such an extruder is a standard commercially available 38 mm extruder Betol, Luton, England. The inlet for water injection should be at an intermediate point, typically between zones 2 and 3, but possibly at a later stage. There should be sufficient processing time after injection for the water to be intimately mixed with the blend before reaching the die. In a preferred embodiment, the die is treated with a PTFE non-stick coating. The die may be of any appropriate geometry depending on the nature of the extrudate required, e.g. a ribbon, rod or film. Paniculate material for use in packaging and insulation can be obtained by passing the extruded ribbon through a rotary cutter. The extruded product typically has a cell size of 0.1 mm to 8 mm and a bulk density similar to that of expanded polystyrene. This material can be used for all the purposes where expanded polystyrene is typically useful but possesses the following distinct advantages: 1. The material is anti-static. 2. It has a higher strength 3. The product has an excellent conformation memory, being able to regain its original shape after compression better than expanded polystyrene. 4. The material has a much lower fire risk, being less inflammable and producing less toxic fumes when burnt. 5. The material is totally compostable, hot water soluble or cold water soluble as required, and recyclable into adhesives etc. 6. It is possible to add standard fire retardant additives to render the material totally fireproof. 7. The product has excellent insulation properties, typically having a U-value similar to that of other expanded foams. In a preferred embodiment of the invention, the blend can be modified by supplementing/replacing part of the polyvinyl alcohol with a compatible resin capable of curing under irradiation, e.g. by UV light. Typical materials are resins used in the photo-resist field, in particular cinnamic esters such as polyvinyl cinnamate, or alternatively adding an irradiation-activated curing agent for the polyvinyl alcohol, for example an alkaline metal or ammonium dichromate or a proprietary diazo compound. In this modification, the blend is extruded as described above, cooled and then subjected to irradiation, typically by UV light in an appropriate chamber. The time of exposure and the levels of curing material included in the composition will determine the degree of surface cure. If only the surface is required to cure, the composition can be made opaque to UV light by appropriate additives. Thus, in this embodiment, the product is obtained with a cured outer coating which renders the product completely impermeable. It will be recognised that this is a significant advance over standard polyvinyl alcohol films which have a permeability which is subject to variation with the relative humidity of the surrounding atmosphere. Typical polyvinyl alcohol products tend to be impermeable under dry conditions but permeable under humid conditions. For many applications, this is undesirable. In a further modification of the present invention, the UV-curing treatment can be applied to a polyvinyl alcohol product, particularly an extruded film, which is not expanded. In this case the production is as described above, without the intermediate injection of water. In this case, the extruder is run at higher intermediate and final temperatures and the barrel is vented, in order to evaporate much of the water and glycerol before extrusion to prevent bubbles forming in the extrudate, typically at zone temperatures (as defined above) in the region of 140, 190, 230, 210, 210 and 233°C respectively. The product, in this case, is a material with an exterior which is impermeable to gases and water, but which is nevertheless biodegradable and hot water soluble once the surface is disrupted, for example by mechanical abrasion or shredding. In an alternative embodiment, the UV-curable component, typically polyvinyl cinnamate, may be applied to the extruded product as a spray or dip coating using an aqueous solution and subsequent irradiation. In a less preferred embodiment, the extruded product could be treated with an active derivative of cinnamic acid, e.g. cinnamoyl chloride in the presence of a base such as pyridine, to esterify the surface. The following examples illustrate the invention further: Example 1 Hot Water Soluble Blown Film Materials 50:50 blend Mowiol ® 56-98, 28-99 (Hoechst) 25% Glycerol 10% Water 0.25% Silica R972 Colouring agent (if required) Place all materials into Baker Perkins Hydisperser (or other similar mixer), mix on full speed until temperature in chamber reaches 109°C. Transfer mix into a cooler and cool until temperature of blend reaches 30°C. Place materials in the hopper of an extruder using 2: 1 compression general purpose screw. The barrel has water injection system at or near zone two for increased foaming properties. Typical profile: Zone 1 Zone 2 Zone 3 Zone 4 adapter die 100°C 120°C 130°C 130°C 130°C 135°C The screw speed should be set at maximum (typically 75 rev. min " ). The temperatures and speed are adjusted for optimum performance depending upon size of machine. The above profile is based on 38 mm extruder (Betol). Example 2 Waterproof, Hot Water Soluble Blown Film The procedure of Example 1 was repeated, but including the step of spray cooling the extrudate with an aqueous 5% solution of ammonium dichromate and subsequent exposure to UV to produce a surface-cured product which is effectively waterproof but which can be rendered hot water soluble by being shredded or abraded. Example 2 Cold Water Soluble/waterproof film Materials Mowiol 56-88 (Hoechst) Glycerol 15% of Mowiol weight Water 5% of Mowiol weight Silica R972 0.25% of Mowiol weight Polyvinyl cinnamate 50% of Mowiol weight Place all ingredients into a Baker Perkins mixer and run on full speed until the temperature in the chamber reaches 110°C. Transfer the blend to a cooler and cool to 30°C. Place the plasticised composition into the hopper of Kytron Soder twin screw feeder arranged to supply an extruder with a vented barrel and a screw of an appropriate type for polyvinyl alcohol. Temperature profile from hopper to die (°C) Zone 1 Zone 2 Zone 3 Zone 4 Adaptor Die 140 190 230 210 210 233 The Kytron vernier is set at 5.6 with 3: 1 screws. The extruder screw speed is set at 20 rev. min "1 . The adaptor is a 10 mm diameter extruder (Betol, Type BC38). The die may be either sheet or blown film type. After film extruded it is cooled in conventional manner and passes through an ultra violet chamber for subsequent exposure. The time of exposure together with relative levels of polyvinyl cinnamate present within the mix will determine the "coating" thickness within the Enpol film.

Claims: CLAIMS: 1. A process for the production of a foamed polyvinyl alcohol composition comprising the following steps: a) blending one or more finely divided polyvinyl alcohol formulations, in particular compositions having a mol % hydrolysis figure of 74 to 94, e.g. 80 to 90 for a cold water soluble product or 98 to 100, e.g. about 99 for a hot water soluble product with a minor proportion of glycerol and water, optionally with colouring and opacifying agent or the like, the blend being allowed to reach a temperature of at least 100; b) passing the blend through a screw extruder and extruding the material at a die temperature of 125 to 240°C, the blend having been saturated with water, either by simultaneously injecting water at an intermediate point in the extruder barrel, or by first soaking the blend in water and then drying the surface before introduction into the extruder. 2. A process according to Claim 1, in which the blend is modified by supplementing/replacing part of the polyvinyl alcohol with a compatible resin capable of curing under irradiation, or alternatively adding an irradiation-activated curing agent for the polyvinyl alcohol, or the UV-curable component or curing agent is applied to the extruded product as a spray or dip coating using an aqueous solution and subsequent irradiation. 3. A process for the production of an extruded polyvinyl alcohol composition in which a blend of polyvinyl alcohol(s) and plasticisers is extruded characterised in that the blend is modified by supplementing/replacing part of the polyvinyl alcohol with a compatible resin capable of curing under irradiation, or alternatively adding an irradiation-activated curing agent for the polyvinyl alcohol or the UV-curable component or curing agent is applied to the extruded product as a spray or dip coating using an aqueous solution and subsequent irradiation.

Inventors: TAYLOR PAUL, TAYLOR, PAUL
Applicants: POLYVAL PLC, TAYLOR PAUL, TAYLOR, PAUL
Application year: 1997
Main classification: C08J9
IPC codes: C08J9, C08K5