Film-Grade LLDPE Comonomer Insertion: Butene-1 as Short-Chain Branching Agent

Gas-phase and solution polyethylene units consume polymer-grade 1-butene as a comonomer in linear low-density polyethylene (LLDPE) and selected high-density polyethylene (HDPE) resins. The monomer is introduced as a vapor into the fluidized-bed reactor, with the comonomer-to-ethylene molar feed ratio maintained between 0.05:1 and 0.20:1 depending on target density and melt index. On-line gas chromatography controls the C4/C2 ratio to a tolerance of ±0.005 mol/mol, because small variations shift chain branching frequency and therefore film toughness. Gas-phase trains operate at 80°C to 110°C and 1.5 MPa to 3.0 MPa total pressure with Ziegler-Natta or metallocene catalysts. In solution processes using cyclohexane or a light hydrocarbon solvent, polymerization temperature is typically 150°C to 220°C, and 1-butene is fed as a liquid; this allows higher comonomer incorporation without fouling. The incorporation of 1-butene inserts an ethyl branch into the chain backbone. At 0.5 to 3.5 mol% comonomer content, the ethyl branches reduce lamellar thickness and crystallinity, lowering density from 0.940 g/cm³ toward 0.915 g/cm³ and broadening sealing temperature. Density is measured on compression-molded specimens according to ASTM D1505-18 or ISO 1183-1:2019; melt mass-flow rate is determined at 190°C under 2.16 kg load per ASTM D1238-20 or ISO 1133-1:2022.

Blown-film conversion of butene-LLDPE is performed on extruders with throughputs from 200 kg/h to 800 kg/h, die diameters of 200 mm to 500 mm, and blow-up ratios of 2.0:1 to 3.5:1. Bubble stability is resin-specific; compared with hexene-LLDPE at equal density, butene grades generally show lower dart impact and tear strength. Representative technical data sheet ranges for 25 µm blown film in the 0.918–0.925 g/cm³ density band are shown below. These are not universal values; they depend on die gap, frost-line height, melt temperature, and film gauge uniformity.

Regulatory compliance for food-contact film made from butene-LLDPE falls under FDA 21 CFR 177.1520 for olefin polymers, provided the resin meets extractables limits established in the applicable clearance. In the European Union, the finished film is assessed against Regulation (EU) No 10/2011 and its amendments, with overall migration limited to 10 mg/dm² of food contact surface. Under REACH, 1-butene monomer is registered as a substance and must be handled within its assigned exposure scenario. Terminal articles include cast stretch film, agricultural silage film, frozen food packaging, heavy-duty shipping sacks, can liners, and geomembrane cushion layers. Each terminal article requires a different balance of density, melt index, and comonomer content; a frozen-food film prioritizes low-temperature seal and dart impact, while a heavy-duty sack prioritizes tensile strength and puncture resistance at the expense of clarity.

In isotactic polybutene-1 production, 1-butene is fed as a high-purity monomer to bulk or slurry polymerization trains using stereospecific Ziegler-Natta catalysts. Monomer specification for this route typically requires 1-butene purity above 99.0 wt%, with isobutylene limited to below 0.5 wt%, water below 5 ppm, and oxygen below 1 ppm to avoid catalyst deactivation and atactic fraction generation. Polymerization is run at 50°C to 70°C and 0.5 MPa to 1.5 MPa in liquid monomer or an inert hydrocarbon slurry. Hydrogen is metered as chain-transfer agent to control melt mass-flow rate in the 0.4 to 20 g/10 min range at 190°C/2.16 kg. The resulting isotactic homopolymer displays a delayed solid-state phase transition after melt processing. The first crystalline form produced on cooling is the metastable tetragonal Form II with a density near 0.890 g/cm³. Over 5 to 10 days at ambient temperature, Form II transforms to the stable hexagonal Form I, increasing density to 0.914–0.917 g/cm³, raising flexural modulus and hardness, and generating measurable post-crystallization shrinkage. This transition is not a defect; it is intrinsic to PB-1 and must be accommodated in pipe dimensioning and packaging-film winding. Extrusion of PB-1 pipe is carried out at melt temperatures of 220°C to 250°C, with barrel zones set from 180°C to 230°C; residence time above 260°C is kept below 10 min to suppress thermo-oxidative chain scission. Calibration and haul-off rates are adjusted so that the slow Form II→Form I transformation completes without excessive internal stress. Under ISO 15876-1:2017, PB-1 piping systems are specified for hot- and cold-water installations; long-term hydrostatic strength is evaluated according to ISO 9080 at 20°C, 60°C, 70°C, and 95°C. The pipe’s design stress is not fixed by short-term yield alone but by the lower confidence limit of the long-term hydrostatic strength regression curve at 50 years.

Injection molding of PB-1 fittings uses melt temperatures of 210°C to 250°C and mold temperatures of 20°C to 80°C. The slow crystalline transformation causes post-molding dimensional change; therefore, dimensional acceptance checks are postponed until after phase conversion or a standardized accelerated conditioning procedure is applied. PB-1 pipe and fittings are tested for hydrostatic strength at 20°C, 70°C, and 95°C per ISO 15876-2:2017, and for resistance to slow crack growth under long-term pressure. A compliance matrix for this application is tabulated below.

After complete Form I transformation, PB-1 homopolymer typically shows tensile yield stress between 18 MPa and 22 MPa per ISO 527-2:2012, elongation at break above 200%, flexural modulus between 350 MPa and 500 MPa per ISO 178:2019, and thermal conductivity near 0.22 W/m·K. These values position PB-1 between polyethylene and polypropylene in stiffness, but the material retains high creep resistance and hoop stress retention in hot-water service. Food-contact PB-1 grades are evaluated under the same olefin polymer regulatory framework as polyethylene: in the United States, FDA 21 CFR 177.1520 applies to polybutene-1 homopolymers and butene-1 copolymers; in the European Union, the material is assessed under Regulation (EU) No 10/2011 and its amendments. The final articles are installed in hot- and cold-water plumbing networks, underfloor heating circuits, district heating manifolds, compressed air distribution, and packaging films requiring toughness. In each of these installations, the slow Form II→Form I transformation is not a post-processing fault but a design parameter; if pipes are pressure-tested immediately after extrusion, the measured modulus and tensile strength can differ from final service values by more than 20%.

Why Does sec-Butanol Dehydrogenation Impose a Purity Limit on 1-Butene Feeds?

sec-Butanol synthesis based on 1-butene is a two-step value chain: acid-catalyzed hydration of n-butenes to sec-butanol, followed by catalytic dehydrogenation to methyl ethyl ketone (MEK). The feed is normally a mixed C4 raffinate stream after 1,3-butadiene extraction and isobutylene removal; the 1-butene fraction is not isolated to polymer grade because the hydration catalyst tolerates the presence of 2-butene and butanes. Direct hydration over solid acid or heteropolyacid catalysts operates at 150°C to 180°C and 2.0 MPa to 4.0 MPa, with a water-to-olefin molar ratio of 2:1 to 5:1. The equilibrium conversion of n-butenes to sec-butanol under these conditions is below 10% per pass; therefore, commercial units recycle the unreacted C4 stream through the reactor after separating the aqueous alcohol phase. Accumulation of inert butanes is controlled by a purge stream, while isobutylene must be removed upstream because it hydrates to tert-butyl alcohol and contaminates the subsequent dehydrogenation section. The catalytic dehydrogenation of sec-butanol to MEK is endothermic and is carried out over copper-chromite or copper-zinc oxide catalysts at 250°C to 300°C and near atmospheric pressure. Per-pass sec-butanol conversion is commonly held at 80% to 90%, with MEK selectivity above 92%; unconverted sec-butanol is recycled through the reactor, and light by-products such as unreacted sec-butanol, hydrogen, and small amounts of methyl isobutyl ketone are separated by distillation. The final MEK product is specified under ASTM D740, with a purity above 99.5 wt%, water content below 0.1 wt%, and acidity below 0.01 wt% as acetic acid. These purity limits are operationally important because residual sec-butanol or water in MEK interferes with polyurethane adhesive curing and surface coating drying.

Terminal use of MEK from butene-based sec-butanol spans solvent-borne polyurethane adhesives, thermoplastic polyurethane coatings, acrylic and vinyl resin lacquers, degreasing operations in electronic fabrication, and dewaxing of lubricating oil base stocks. The solvent is regulated as a volatile organic compound in industrial emissions; in the European Union, the substance is registered under REACH, and workplace exposure is controlled under the applicable national occupational exposure limit. Because MEK is highly flammable, closed-cup flash point is -9°C, and storage requires flameproof electrical classification in process areas. No food-contact claims apply to the solvent itself; food-contact relevance is limited to the final polymer or coating after solvent removal and cure.

Refinery C4 alkylation units consume 1-butene as part of a mixed butene stream after butadiene extraction and optional isobutylene removal, reacting it with excess isobutane to produce high-octane branched paraffins. In sulfuric acid alkylation, the emulsion reactor is held at 4°C to 10°C with an isobutane-to-olefin volumetric ratio of 6:1 to 15:1; in hydrofluoric acid alkylation, the reactor temperature is 27°C to 43°C and the same excess of isobutane is maintained to suppress oligomerization. 1-Butene entering the acid zone can undergo double-bond isomerization to 2-butene, and the resulting alkylate selectivity depends on the rate of hydride transfer from isobutane to the adsorbed C4 carbocation. Excessive 1-butene concentration without sufficient isobutane favours C8+ oligomer formation and acid consumption, reducing trimethylpentane selectivity and lowering research octane number. Commercial alkylate from mixed n-butenes typically exhibits a research octane number of 90 to 96 according to ASTM D2699-21 and a motor octane number of 88 to 93 according to ASTM D2700-21. Reid vapour pressure is controlled below 20 kPa when measured by ASTM D5191-20, making alkylate a low-sulphur, low-RVP gasoline blendstock. The alkylate is blended into motor gasoline at 10% to 60% by volume depending on refinery pool constraints, oxygenate limits, and sulphur specifications under ASTM D4814-21 or EN 228:2012+A1:2017.

The terminal product of this application is not a discrete polymer but a gasoline blendstock that reduces the pool’s sulphur, olefin, and benzene content. Because alkylate from 1-butene has a lower research octane number than alkylate from isobutylene, refineries that rely on 1-butene-rich feeds often direct the stream to a double-bond isomerization step before the alkylation reactor. Isomerization of 1-butene to 2-butene over an acidic fixed-bed catalyst improves trimethylpentane formation, but requires drying of the feed to prevent oxygenate formation and acid dilution. Alkylate produced under these conditions is also used in aviation gasoline blends where a minimum motor octane number and low vapour pressure are mandatory. No food-contact regulatory status applies to alkylate; its specifications are defined by fuel standards and pipeline custody transfer requirements, including ASTM D4057 for sampling and ASTM D86 for distillation.

When Ethylene Meets Isomerized Butene: Metathesis Reactor Constraints

The conversion of 1-butene to propylene by olefin metathesis requires a two-step pathway: double-bond isomerization of 1-butene to 2-butene, followed by cross-metathesis of 2-butene with ethylene in a fixed-bed reactor. The process is used in refineries and olefin plants where polymer-grade 1-butene is not economically recovered or where propylene demand exceeds steam cracker output. Isomerization is carried out over basic or acidic catalysts at 100°C to 300°C; magnesium oxide, potassium-promoted alumina, or zeolitic materials convert 1-butene to an equilibrium mixture of cis-2-butene and trans-2-butene. The metathesis reactor operates at 300°C to 400°C and 1.0 MPa to 3.0 MPa over tungsten oxide on silica or molybdenum oxide on alumina. An ethylene-to-2-butene molar ratio above 1:1 is maintained to shift the equilibrium toward propylene and suppress butene self-metathesis. Reported per-pass butene conversion in commercial olefin conversion units is approximately 60% to 70%, with propylene selectivity above 90%; published data for this specific configuration is limited, and licensor performance varies with catalyst cycle age and feed dilution. Feed pretreatment is critical: water is reduced to below 0.1 ppm, oxygen to below 1 ppm, and sulphur to below 1 ppm to prevent acid-site poisoning and carbon deposition. The propylene product is distilled to polymer-grade specifications; propylene purity is measured by gas chromatography according to ASTM D2712-20, with methylacetylene and propadiene limited to below 5 ppm each for polypropylene catalysis.

The terminal articles made from metathesis-derived propylene include polypropylene homopolymers and copolymers, propylene oxide, acrylonitrile, acrylic acid, and cumene. In each route, the critical property is not the source of the propylene but its impurity profile; even trace levels of butene, ethylene, water, or dienes alter metallocene or Ziegler-Natta catalyst activity and final resin molecular weight distribution. For polypropylene film and pipe grades, melt flow rate is determined according to ISO 1133-1:2022 or ASTM D1238-20 at 230°C under 2.16 kg. The metathesis route does not require food-contact approval for 1-butene as such, because 1-butene is not present in the final polypropylene article; compliance is assessed on the finished resin under FDA 21 CFR 177.1520 or Regulation (EU) No 10/2011 as applicable.

Butene Oligomerization to Isononyl Alcohol Precursors

Raffinate-2 streams containing 1-butene and 2-butene are oligomerized in liquid phase over nickel-based coordination catalysts to yield a mixture of C8 and C12 olefins. The Dimersol X process and similar fixed-bed or homogeneous nickel systems operate at 20°C to 50°C and 0.5 MPa to 1.5 MPa. Per-pass butene conversion is approximately 70% to 80%, with C8 selectivity above 85%; the C8 olefin stream is then subjected to hydroformylation with syngas over cobalt or rhodium catalysts to form isononyl alcohol. The hydroformylation step operates at 120°C to 200°C and 5.0 MPa to 15.0 MPa depending on catalyst type; the aldehyde intermediate is hydrogenated and distilled to produce isononyl alcohol with a purity above 99.5 wt%. Esterification of isononyl alcohol with phthalic anhydride or trimellitic anhydride yields diisononyl phthalate (DINP) or triisononyl trimellitate (TOTM), both used as PVC plasticizers. The plasticizer esters are tested for density, viscosity, and volatility by standard methods; for example, kinematic viscosity is measured by ASTM D445-21, density by ASTM D4052-20, and acid number by ASTM D974-21.

The terminal articles from this butene-based chain include flexible PVC flooring, wire and cable jacketing, automotive interior cover stock, coated fabrics, and low-temperature flexible tubing. Regulatory compliance for the plasticizer in food-contact or medical applications is not automatic; it depends on specific migration limits in the relevant jurisdiction. In the European Union, DINP used in food-contact materials is assessed under Regulation (EU) No 10/2011 and its amendments, with specific migration limits defined for the final article. In the United States, plasticizer acceptance is addressed through food additive regulations or food contact notification pathways rather than the olefin polymer regulation; no claim of FDA clearance applies to the plasticizer itself. Because residual butene and C8 olefins are removed from the alcohol before esterification, the final plasticizer contains no detectable 1-butene monomer. Process water and catalyst residues are managed under REACH and local waste-water permits; nickel-containing catalyst is recovered or disposed of as hazardous waste according to the applicable national legislation.