POLYURETHANE PREPOLYMER

The present invention relates to a method to produce polyurethane (PU) prepolymer from biopolyol, selected from the group consisting of palm kernel oil-based polyol (PKO-p), coconut oil-based polyol (CO-p) and soybean polyol (SB-p).

BACKGROUND ART Polyurethane (PU) products have many uses and are widely used in various industries. The PU produced herein is targeted for the use in coating, adhesive and sealant industry but is also applicable for solar cells, as semiconductor substrate to replace glass and other PU industries. PU are block copolymers containing segments of low molecular weight polyester or polyether bonded to a urethane group (-NHCO-0). Traditionally, these polymers are prepared by reacting three basic materials; polyisocyanates, hydroxyl-containing polymers (polyester or polyether polyol) and chain extender, normally low molecular weight diol or diamine (such as 1,4-butanediol or 1 ,4-dibutylamine).

At present most polyols used in polyurethane industry are petroleum-based where crude oil and coal are used as starting raw materials. However, these materials have been escalating in price and rate of depletion is high as well as required high technology processing system. This necessitates a look at utilizing plants that can serve as alternative feed stocks of monomers for the polymer industry. Moreover, with increasing annual consumption of polyurethane, its industrial waste is a serious matter.

In the effort towards achieving sustainable manufacturing, many non-petroleum resources have been researched as green material such as plant oil polyols such as vegetable oils like palm oil, soybean oil, coconut oil, sunflower oil, olive oil and canola oil and combined with green technology to produce useful polymers such as PU.

PU can be produced via different methods. A prior art described a simple one step process of producing PU polyester via esterification process with condensation throughout. In general, single step polymerization takes place when polyol, diisocyanate and catalyst are mixed together and chain extension takes place in one step. As a result, heat is liberated. Another conventional prior art describes the production of PU via quasiprepolymer method. This method is the reaction between polyol reacted with an excess of diisocyanate where the urethane prepolymer contains higher free isocyanate content known as isocyanate quasiprepolymer or semiprepolymer where the diisocyanate molecules are partially reacted with polyol.

Malaysia patent MY-145094-A described a process for preparing natural oil-based polyesters and polyamides. Palm kernel oil or coconut oil is processed in two steps involving polyesterification and polycondensation process. It involves adding of palm kernel oil or coconut oil to a mixture comprising polyhydric alcohol. Polysaccharide alcohol catalyst is added to improve the process.

US patent US 6,399,698 provide an isocyanate prepolymer for making polyurethanes. The prepolymer is synthesized by combining epoxidized natural oils with isocyanates and a catalyst. The prepolymer contains oxazolidone rings that can react with polyols to form polyurethane.

US patent US 8,293,808 disclosed a polyurethane foam prepared by reacting in presence of a blowing agent, a polyisocyanate with an active hydrogen containing composition that includes a modified vegetable oil-based polyol. The treatment involved modifying the double bond of the vegetable oil.

Prior arts focus on conventional method to produce PU from extracted plant-based polyols. In the single step polymerization method, the process gives off heat when urethane bonds were formed and causes shrinkage problem while isocyanate vapor levels are present. The present invention is made in view of the prior arts described above where the proposed method, the prepolymerization method, allows for lower isocyanate vapour levels and reduction of exotherm of the final reaction as no heating is carried out. This method also allows for partial dissipation of the exorthem prior to the formation of PU. SUMMARY OF INVENTION

This invention discloses a method of producing prepolymer for PU, without heating process, using vegetable oil bio-polyol selected from the group consisting of palm kernel oil-based polyol (PKO-p), coconut oil-based polyol (CO-p) and soybean polyol (SB-p). The invention utilizes plant-based material which potentially allows for lower production cost and sustainable manufacturing, without dependency on petroleum- based material.

Bio-polyol, such as PKO-p in solution form is mixed with diisocyanate under nitrogen gas atmosphere, said PKO-p to diisocyanate in weight ratio of about 0.75:1 to 2:1 to form urethane prepolymer. PKO-p was diluted in industrial grade solvents selected from the group consisting of tetrahydrofuran (THF) solution in 15 to 35 % (w/w), industrial grade acetone in 3-35% (w/w) and dimethyl sulfoxide in 8-15% (w/w). The diisocyanate is selected form the group consisting of 2,4'-diphenylmethane diisocyanate (2,4'-MDI) or 4,4'-diphenylmethane diisocyanate (4,4'-MDI) or mixtures of both isomers. The diisocyanate can possibly be in its presence form or diluted in the selected industrial grade solvent. The solvent for diisocyanate, if used, must be of the same type of solvent that dilute PKO-p. The PKO-p in solvent and the diisocyanate in solvent are known as prepolymers.

The prepolymers are agitated, solution-casted and cured to form PU. The PU produced via this invention is a clear, either semi-flexible or flexible film that can be used in coating, adhesive, sealant and elastomer industry, especially as coating onto fiberboard due to its hydrophobic properties. The usage is also applicable for solar cells as semiconductor elastomeric substrate to replace glass as well as for other PU industries. BRIEF DESCRIPTION OF DRAWINGS

Fig. 1 is a figure showing the general structure of polyurethane.

Fig. 2 is a figure showing the structure of PKO-p polyol.

Fig. 3 is a figure showing the structure of diisocyanate (4,4'-MDI).

Fig. 4 is a figure showing the structure of urethane polymer with isocyanate end group. Fig. 5 is a figure showing the structure of polyurethane synthesized according to the invention.

Fig. 6 is a figure showing the repeating unit in the structure of polyurethane synthesized according to the invention.

DESCRIPTION OF EMBODIMENTS

Hereinafter, the present invention is described in detail.

Polyurethane (PU) (20) is based on polyol. Preparation of proposed PU (30) via prepolymerization involves the formation of urethane polymer at the initial stage of reaction between polyol (22) and diisocyanate (24). Then, the urethane prepolymer (26) may be chain extended with diol, or any other hydroxyl containing substance, and cured to form PU (30). During the urethane prepolymerization (26), one of the isocyanate (24) groups, NCO will react with a hydroxyl group, OH of the polyol (22) and the other isocyanate (24) group reacts with the second OH group. The resultant urethane prepolymer has two isocyanate groups at both ends (28) of urethane prepolymer (30) with urethane bonds. This creates new properties to the polyurethane backbone as compared to conventional method (20) where the former still maintaining the same repeating unit (32). Both prepolymers are diluted in industrial grade solvents selected from the group consisting of tetrahydrofuran (THF) solution in 15 to 35 % (w/w), industrial grade acetone in 3-35% (w/w) and dimethyl sulfoxide in 8-15% (w/w).

Example 1

Based on the above principles, PKO-p is used as the renewable raw material where prepolymerization method is carried out. The solvent used may be tetrahydrofuran (THF), acetone, dimethyl sulfoxide (DMSO) or N-methy-2-pyrrolidone (NMP) while the chain extender can be any range of glycol selected from the group of monoethylene glycol, diethylene glycol, and polyethylene glycol of molecular weight ranging from 200 to 20,000. The diisocyanate are 2,4'-diphenylmethane diisocyanate (2,4'-MDI) or 4,4'- diphenylmethane diisocyanate (4,4'-MDI).

Example 2

The PKO is initially dissolved in THF before mixing with MDI in a round bottom flask under nitrogen gas atmosphere at ambient temperature to form urethane prepolymer (26). DEG is then added at varying amount to the PKO. The mixture is then agitated at 200 rpm for an hour at room temperature. The mixture is then casted into a translucent film of about 50 microns thickness onto a Teflon plate. The film is then dried in a vacuum oven at 35-55°C for 12-24 h to remove the solvent. The final produced PU (30) is yellow, translucent and void-free films.

Example 3 PKO-p is reacted with 2,4'-MDI in 5-35 mL acetone at varying NCO/OH weight ratio of 100/100, 100/150, 100/200 and 100/250. The reaction is conducted at ambient temperature under nitrogen gas atmosphere in a glass flask.

Example 4

The pre-polymer solutions are stirred to a homogenous solution for 5 min and the solution was casted to a teflon mould and allowed to evaporate at room temperature for an hour. The produced films were kept in a desiccator for the further characterization. The final cured films showed PU are from rigid and brittle to semi-rigid and flexible films. Curing can be performed at mild condition.

The chemical route of producing PU (30) via this prepolymerization method in this invention is as follows and the corresponding chemical structure for PU (30), polyol (22), isocyanate (24) and urethane polymer (26) with isocyanate end group (28) is shown as Figure 1 to Figure 6, respectively.

Via this prepolymerization method, the final PU (30) is consistent where all MDI has completely reacted to form PU (30) and the urethane bond is the main polymeric chain in the PU (20) as indicated by the hydrogen bonding formed between soft segmented PKO-p chains with hard segmented MDI.

Accordingly, a method of producing environmentally friendly PU (20) via prepolymerization method without heating process using renewable vegetable oil is disclosed. Prepolymerization is conducted for PKO-p (22) with MDI for diisocyanate component, acetone as the solvent and DEG as chain extender in varying ratios under nitrogen gas atmosphere at ambient temperature. The PU produced via this invention is clear and semi flexible film that can be used in coating, adhesive and sealant industry especially as coating onto fiberboard due to its hydrophobic properties, solar cells, as semiconductor substrate to replace glass and other PU industries.