Determination of Dimensional Change Rate of PVC Flooring: Test Principles, Procedures, and Standards

The dimensional change rate test is a core quality control procedure for PVC flooring (PVC floor leather), quantifying how much the material permanently shrinks or expands after exposure to elevated temperature. Excessive dimensional change is a leading cause of seam opening, gapping, curling, and buckling in installed vinyl floors, especially over underfloor heating. This article explains the test principles, sample requirements, equipment, step-by-step procedure, calculation method, and acceptance criteria based on standards such as EN ISO 23999 and GB/T 11982.

Technical Overview

PVC flooring is a resilient floor covering produced by calendering, extrusion, or coating processes. During manufacturing, molecular orientation and frozen-in internal stresses are introduced into the material, and the plasticizer system continues to equilibrate over time. When the finished floor is later exposed to elevated temperatures, such as sunlight through windows, underfloor heating systems, or hot service water pipes, these internal stresses relax and the material undergoes a permanent change in dimensions.

The dimensional change rate, also referred to as dimensional stability, is defined as the percentage change in the distance between two reference marks on a conditioned test specimen after a standardized heat treatment, relative to the original distance between the marks. It is one of the most important performance indicators for PVC flooring because excessive dimensional change in service causes visible joint separation, gaps between sheets or planks, edge lifting, curling, and buckling of the floor surface.

The test is internationally standardized. The principal method is EN ISO 23999, Resilient floor coverings - Determination of dimensional stability, which superseded the earlier EN 434. In China, the corresponding requirements are specified in GB/T 11982.1 for polyvinyl chloride floor sheets and GB/T 4085 for semi-rigid PVC sheet flooring. Although the wording differs slightly, these standards follow the same essential principle: condition the specimen, measure the initial gauge length, expose the specimen to 80 °C for 6 hours in a ventilated oven, recondition it, remeasure, and calculate the percentage change.

Testing Samples

Homogeneous PVC Flooring

Homogeneous PVC flooring is a single-layer product with a uniform composition through its full thickness. Because the entire structure is stressed during calendering, dimensional change tends to be more pronounced and more anisotropic than in reinforced products, and shrinkage in the machine direction usually deserves the closest attention.

Heterogeneous (Multi-Layer) PVC Flooring

Heterogeneous products consist of several layers, typically a transparent wear layer, a printed decoration layer, a foamed intermediate layer, and a backing layer. Differential thermal response between layers can generate not only in-plane dimensional change but also curling. For this reason, most heterogeneous products incorporate a glass fiber mesh during manufacture to mechanically restrain dimensional change.

PVC Floor Leather in Roll Form

For roll goods, samples should be taken at least 1 m away from the end of the roll to avoid non-representative material. Three specimens, each typically 250 mm × 250 mm, are cut with their edges precisely parallel and perpendicular to the machine direction of the roll. Each specimen must be clearly labeled to identify the machine direction (MD) and the transverse direction (TD), and results for the two directions must never be averaged together.

Specimen Preparation and Conditioning

• Cut specimens cleanly without stretching, creasing, or bending them, and remove any burrs along the edges.
• Mark pairs of fine reference lines approximately 200 mm apart in each direction on the surface of the specimen, using a sharp scriber or a fine-tipped marker, so that the gauge length can be reproduced accurately.
• Condition all specimens in a standard laboratory atmosphere of 23 ± 2 °C and 50 ± 5 % relative humidity for at least 24 hours before the initial measurement.
• Handle specimens only by their edges and keep them lying flat on a rigid surface at all times to avoid introducing external stresses.

Testing Items

• Dimensional change rate in the machine direction (length direction of the roll).
• Dimensional change rate in the transverse direction (width direction of the roll).
• Supplementary visual assessment of flatness, curling, and edge distortion of the specimen after heat exposure.

Gauge lengths are measured with a device accurate to 0.05 mm, such as a graduated steel scale with a vernier, a measuring microscope, or an optical projector. In most specifications derived from EN ISO 23999, the dimensional change rate in each direction is required not to exceed 0.25 %, although individual project or purchaser specifications may impose different limits. The sign of the result is meaningful: a positive value indicates expansion, while a negative value indicates shrinkage, and both directions are reported separately for each specimen together with the arithmetic mean.

Testing Methods

Equipment Required

• A forced-air circulation oven capable of maintaining 80 ± 2 °C with a uniform temperature distribution over the specimen area.
• A conditioning chamber or controlled laboratory environment at 23 ± 2 °C and 50 ± 5 % relative humidity.
• A length measuring instrument with an accuracy of 0.05 mm or better.
• A flat, rigid support plate and a level bench free from vibration.

Test Procedure

• Condition the prepared specimens for at least 24 hours in the standard atmosphere, then measure and record the initial gauge lengths in both directions to the nearest 0.05 mm as the original lengths.
• Place the specimens flat and free of any restraint inside the oven at 80 ± 2 °C, spaced apart so that air can circulate freely around each specimen and no specimens touch one another or the oven walls.
• Maintain the heat exposure for 6 hours, keeping the oven door closed throughout the period.
• Remove the specimens carefully without bending them, allow them to cool freely at room temperature, and recondition them in the standard atmosphere for at least 24 hours.
• Measure the final gauge lengths in both directions using the same reference marks and the same measuring technique as for the initial measurement.
• Calculate the dimensional change rate for each direction of each specimen.

Calculation of Results

The dimensional change rate is calculated using the following formula: the rate equals the difference between the final gauge length and the original gauge length, divided by the original gauge length, multiplied by 100 percent. A positive result indicates that the specimen expanded during the heat treatment, while a negative result indicates shrinkage. The individual values and the arithmetic mean are reported separately for the machine direction and the transverse direction, normally rounded to 0.01 %, and the mean values are compared against the specified limit.

Key Precautions

• Specimens must lie completely free and flat during heat exposure; any restraint will produce falsely low results.
• The oven must be calibrated and verified before testing, and specimens should not be placed in the direct path of radiant heating elements.
• Never bend, stretch, or flex a specimen during transfer between the oven, the conditioning area, and the measuring bench.
• Testing should be performed at a consistent interval after production, for example at least 72 hours, because the internal stress state of freshly calendered material evolves during the first days.
• Reference marks must remain sharp and unambiguous after heat treatment so that both measurements are taken between the same points.

Applications

• Factory quality control, where every production batch is checked to confirm dimensional stability before release.
• Type testing and product certification under national and international standards for resilient floor coverings.
• Qualification of products for underfloor heating systems, conservatories, and sun-exposed commercial interiors where thermal loading is high.
• Research and development, where the test is used to optimize plasticizer dosage, filler loading, glass fiber reinforcement, and calendering or annealing parameters.
• Acceptance inspection for construction projects and compliance verification for import and export shipments.

Because dimensional stability directly governs the long-term appearance of seams and joints in service, the test result is frequently a decisive acceptance criterion in flooring tenders, particularly for hospitals, schools, clean rooms, and transportation vehicles where post-installation repair is costly and disruptive.

FAQ

Why is the specimen heated at 80 °C for 6 hours?

The 80 °C exposure for 6 hours is an accelerated simulation of the thermal stress a floor experiences during transport, storage, installation, and years of service, including solar gain and underfloor heating. It is severe enough to relax essentially all frozen-in processing stresses within a short laboratory time, so the measured change represents the worst-case permanent dimensional movement of the material.

What dimensional change rate is considered acceptable?

Most specifications based on EN ISO 23999 require the dimensional change rate in each direction to be no greater than 0.25 %. Some product classes and project specifications may allow slightly larger values, but lower results are always preferred. A result of 0.25 % corresponds to 0.5 mm of movement over a 200 mm gauge length, which is significant when multiplied over a wide room.

Why do specimens often shrink more in one direction than the other?

Calendering and extrusion orient the polymer chains and entrain directional internal stresses along the machine direction. When heat allows these stresses to relax, the material recovers preferentially along that direction, so the machine direction typically shows larger shrinkage than the transverse direction. The test therefore always evaluates both directions independently.

How can manufacturers reduce the dimensional change rate?

Effective measures include embedding a glass fiber scrim in the product structure, annealing the material after forming to relax processing stresses, optimizing the plasticizer and filler formulation to reduce post-production migration, controlling calendering temperature and draw-down ratios, and allowing adequate maturation time before conversion and installation.

Does this test apply to luxury vinyl tile and plank products?

Yes. Loose-lay planks, glue-down tiles, and click-lock luxury vinyl flooring are all sensitive to dimensional change, and dimensional stability testing is equally important for them. In click-system products, even small dimensional changes can alter joint gaps and affect the mechanical engagement of the locking profiles, so manufacturers routinely apply the same test method during development and batch control.